1. Field of the Invention
The invention relates to gas separations. More particularly, it relates to the enhanced production of oxygen from air.
2. Description of the Prior Art
Adsorption processes have been widely used for the separation and purification of gases. High surface-area sorbents have an affinity for adsorbing gas molecules on the surface thereof. The quantity of gas adsorbed depends on the specific sorbent employed, on the gas being adsorbed, and on the temperature and pressure conditions under which the adsorption operation is carried out. For most sorbents, the quantity adsorbed increases as the partial pressure of the gas component being adsorbed increases and as the adsorption temperature decreases. Thus, the amount of gas adsorbed can be increased by decreasing the adsorption temperature. In most practical applications, it is necessary to desorb the adsorbed gases so as to regenerate the sorbent to enable the adsorption process to be repeated on a cyclic basis. The desorption step proceeds best at high temperatures and low pressures. For such practical applications, therefore, either the pressure or the temperature, or both, must change or "swing" on a cyclic basis between the adsorption and desorption steps. These two basic approaches for gas separation are called pressure swing adsorption (PSA) and temperature swing adsorption (TSA).
In recent years, PSA processes have been developed for the production of oxygen and nitrogen from air. In such processes, feed air is passed to an adsorption bed containing sorbent capable of selectively adsorbing a more readily adsorbable component from air, i.e. either nitrogen or oxygen, while the less readily adsorbable component is discharged from the adsorption bed. While the behavior of such PSA processes is clearly influenced by the temperature conditions under which adsorption and desorption take place, most PSA processes have been designed to operate under generally ambient temperature conditions without the use of specific means for controlling the temperature conditions pertaining to the adsorption operation.
In PSA systems, heat is liberated upon adsorption, and heat is taken up by the sorbent upon desorption. The temperature of the adsorption bed thus tends to rise during the adsorption step, while the temperature of said bed drops during the desorption step. The temperature change is most pronounced during the portion of the overall PSA cycle in which the adsorption bed is being pressurized to an upper adsorption pressure or is being depressurized to a lower desorption pressure, provided that the adsorbent is essentially free of strongly-adsorbed impurities that can only be desorbed effectively by purging and that act to prevent adsorption of less strongly adsorbed components. Pressurization and depressurization of the open gas spaces in an adsorption system, such as the distributor means or headspaces in vessels used to contain the bed of sorbent material, also causes temperature changes by the reversible work done by compression and expansion of gases therein. In a dynamical process such as the PSA process, much of the heat of adsorption and compression is transferred to the flowing feed gas, e.g. air, stream and is carried out of the adsorption bed. In typical PSA processing, such as that used for the production of oxygen and/or nitrogen from air, the forward flow of gas during adsorption exceeds the backward fl the backward flow of gas during desorption. As a result, there is a net flow forward of enthalpy, which tends to reduce the average temperature of the adsorption beds employed in a PSA system when the temperature oscillations therein are greater than in the region near the entrance to the beds.
The effect of temperature on PSA processes for producing oxygen from air is discussed by Izami et al. "High Efficiency Oxygen Separation with Low Temperature and Low Pressure PSA", AIChE, San Francisco, Calif., November, 1989. Five different molecular sieve type sorbents capable of selectively adsorbing nitrogen from feed air were investigated in the reported study, including Na-X (with two different Si/Al ratios), Ca-A, Ca-X, and Si-X. It was found that the sorbents with alkaline earth cations (Ca and Sr) showed the best N.sub.2 /O.sub.2 separation factors at near room temperature, whereas the separation factor peaked for the Na-X sorbents at about -30.degree. C. In all cases, the nitrogen storage capabilities increased as the temperature decreased, as would be expected from adsorption theory as discussed above. Bench-scale process tests with Ca-A and Na-X sorbents confirmed that the Ca-A sorbent performed best between 0.degree. C. and room temperature, whereas Na-X sorbent performed best at temperatures well below 0.degree. C. In these tests, the adsorption beds were thermostated and were effectively maintained at a fixed temperature. Larger-scale pilot tests were also performed with Na-X adsorbent material. Cooling coils were incorporated into the bed, and a heat-regenerator section was also employed between a desiccant section used to dry incoming feed air and the adsorbent bed to achieve bed temperatures lower than that of the feed gas stream. Such tests confirmed that the adsorption efficiency was increased, and the cost decreased, when the adsorption temperature was decreased to a nominal value of -15.degree. C. The tests operated more nearly under adiabatic than isothermal conditions, and the temperatures were not uniform. These tests show that it is advantageous to operate the PSA process with Na-X adsorbent at sub-ambient operating temperatures. External refrigeration was used to achieve the desired low adsorbent bed temperature. An optimum desorption pressure of about 0.3 atmospheres was likewise employed.
Contrary to the above, however, others have found that low adsorbent bed temperatures adversely effect PSA system performance. Collins, U.S. Pat. No. 3,973,931, has disclosed that very large axial temperature variations can occur in superatmospheric PSA processes for producing oxygen from air. Temperature variations of more than 50.degree. C. were observed in adsorbent beds of zeolitic molecular sieve material. A very large temperature gradient was found to be established near the feed end of the bed leading to a temperature minimum at a foot or so from the feed end of the bed, with gradually rising temperatures existing throughout the rest of the bed. After repetitive adsorption-desorption cycling, the temperature profile persisted with only slight variation with each cycle. Collins found that these temperature variation conditions were detrimental to the purity and recovery of oxygen using such superatmospheric PSA cycles. As a result, Collins taught that improved operation results from heating the feed air stream by at least 20.degree. F. (11.degree. C.). Although the operating data presented shows that a large axial temperature variation persists, the minimum bed temperature is thereby raised, as are the temperatures throughout the rest of the adsorbent bed. Collins attributes the inlet end temperature depression to an "inadvertent heat regenerative step" and shows that the temperature depression is greatest when water vapor impurity is being adsorbed from the feed stream in this inlet region of the bed. Collins proposes several means for raising the feed stream temperature, including controlling or partially bypassing the feed air compressor aftercooler. The heat of feed air compression is more than adequate to produce the somewhat higher feed air temperatures used for improved processing in accordance with the practice of the teachings of Collins.
The PSA-air separation art thus contains differing teachings as to the choice of adsorbent materials, the pressure levels for adsorption and desorption, and the recommended operating temperature levels, with temperatures both above and below ambient temperatures being recommended. Nevertheless, as indicated above, most commercial PSA-air separation processes are operated under ambient conditions without temperature control and without particular regard to the heat effects that occur during the cyclic adsorption-desorption operations.
There is, of course, a desire in the art to improve PSA operations so as to more fully satisfy the ever-increasing requirements of practical commercial air and other gas separation operations. Such desire in the art relates particularly to enhancing the recovery of oxygen or other desired products with advantageous PSA systems that utilize rather than disregard the heating effects that occur in the course of cyclic PSA operations. For such enhanced operations, however, it is desirable that the PSA systems avoid the use of relatively expensive auxiliary equipment, such as the external refrigeration employed in accordance with the teachings of Izami et al.
It is an object of the invention to provide an improved PSA process and apparatus for the production of oxygen from air, and other desirable gas separations.
It is another object of the invention to provide a PSA gas separation process and apparatus utilizing the heat effects that occur in the course of the cyclic adsorption-desorption PSA sequence so as to avoid the need for external refrigeration.
It is a further object of the invention to provide a PSA process and system for enhancing the overall efficiency and economy of oxygen production from feed air.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.